The present invention relates to a fiber-based electro-optic system with a probe tip, and polarization control of the beam of light, and in particular, a method and apparatus for scanning a workpiece to be tested using a fiber-based electro-optic system.
Electro-optic (EO) field mapping is becoming recognized as a promising diagnostic measurement technique for the microwave and millimeter-wave regimes. Due to the single-micrometer spatial-resolution, broad bandwidth ( greater than 100-GHz bandwidth), and low invasiveness, EO field mapping has been used for fault isolation of microwave integrated circuits, extreme-near-field mapping and near-to-far-field-transition characterization of antenna elements, performance verification of various active quasi-optical power-combining arrays, and performance testing of active and passive antennas.
A new field-mapping system according to the present invention has been developed using fiber-mounted, micro-machined GaAs crystals as the electric-field sensors, based on the initial embodiment of the EO field-mapping system, known as a xe2x80x9cfree-space measurement systemxe2x80x9d since both the detection and signal laser beams traveled in the open air and for which applications have been limited to exposed, planar structures. This system according to the present invention is much less prone to disturb the device under test (DUT), and provides enhanced flexibility to probe many different structures from a variety of perspectives. Due to the flexibility of the optical fiber and the small size of a micromachined GaAs tip, the fiber-based electro-optic probes may be inserted into enclosures such as waveguides and packages in order to measure electric fields. While a similar concept for a fiber-based field sensor has been recently demonstrated in measurements of 1 GHz microwave signals, the use of such a probe for phase measurements, measurement inside of packages, high-frequency (Ka-band) microwave signal measurements, and the characterization of three orthogonal field components has not been known until the present invention.
An electric-field-mapping technique according to the present invention serves as the foundation for an instrument that will benefit the design, development, production, and quality control of microwave and millimeter-wave antennas, devices, and integrated circuits. The technique senses the electric field from a device under test when the microwave signal modulates a laser beam in a micromachined-GaAs electro-optic crystal. This probe is mounted on an optical fiber in order to provide optimal positioning flexibility and a confined path for the optical beam that returns from the probe with the microwave-field information. Ultrabroadband-field-mapping has been demonstrated for signals of frequencies between 80 MHZ and W band to take advantage of the short-pulse nature of the laser source. The electro-optic sensor according to the present invention is purely a dielectric with no conductive components, as compared with other field probes that use metalized antennas and contain grounded electrodes. Therefore, the probe can be placed very close to the device under test, even into the near field of radiating elements, and extract field information with minimal invasiveness. This is an advantage over every other type of field probe, and it allows the electro-optic sensor to extract the complete electric field information from a device, including evanescent fields and surface modes. Another advantage of the electro-optic probe is the spatial resolution, which at xcx9c5 micrometers leads to high-resolution capabilities that are important for high frequency antenna arrays and for integrated circuits. The electro-optic sensors also are used to isolate the vector electric-field polarizations from a device with a high degree of isolation between orthogonal field components.
The electro-optic probe produces two-dimensional maps of the amplitude and phase of the electric field in any plane above the device under test. It thus provides a capability unique compared to any other measurement instrument for viewing the signals in one part of a microwave circuit or array relative to any other part of the circuit. This capability has also been extended to the interior of enclosed microwave packages, where the probe will be able to diagnose cross-talk and interference between devices and interconnects. The electro-optic probe will be of use in the development or troubleshooting of any design that uses parallel paths where the phase of the electrical signal on one path has a specific relationship to the phase of another signal. The sensor can also be used for fault isolation or failure analysis, as well as validation of electromagnetic models. Since this non-contacting probe has a high impedance and yet is still a broadband sensor, it can also be used to make quantitative S-parameter measurements in certain instances, without the need to de-embed cables or transitions. The fiber-based electro-optic field-mapping technique has been implemented in a working, research-lab, bench-top prototype.
A fiber-based electro-optic field mapping system has been developed using micromachined GaAs probe tips. The fiber-based system has lower permittivity than other scanning field probes, provides excellent measurement flexibility so that the scanning can be performed at any arbitrary orientation, and allows insertion of the field sensor into microwave enclosures and packages. In particular, the fiber-based EO field mapping system makes it possible to extract electric field distributions of complicated micro- and millimeter wave circuits shielded by metal walls. The fiber-based EO system can be applied to the design, characterization, and failure analysis of quasi-optical power-combining arrays, power amplifiers, and other microwave and millimeter wave systems.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.